The Hawkesbury-Nepean Flood Risk Management Strategy

  1. What problem is the Hawkesbury-Nepean Flood Risk Management Strategy addressing?

In December 2012 the NSW Government accepted the recommendation from Infrastructure NSW in the first 20 year State Infrastructure Strategy, to review all major flood mitigation options available, including raising the Warragamba Dam wall, to significantly reduce the potential economic and social impact of flooding in the Hawkesbury Nepean Valley.

The Hawkesbury-Nepean Valley between Penrith and Sackville has the greatest flood risk in NSW. Floods in this Valley pose a significant potential risk to life and property due to the unique topography of the area. A flood similar to that which occurred in Queensland in 2011 would require the evacuation of over 110,000 people and cause damage to thousands homes and businesses costing at least $2 billion.

  1. What is the Hawkesbury-Nepean Flood Risk Management Strategy?

The Strategy is a comprehensive long term plan for the NSW Government, local councils, businesses and the community to manage the risk posed by regional floods in the Hawkesbury-Nepean Valley. They Strategy’s objective is to reduce the potential flood risk to life, the economy and social amenity in the Hawkesbury-Nepean Valley. The Strategy is an integrated mix of infrastructure and non-infrastructure actions which will be implemented both in the short and longer term. This is because no one simple solution or single infrastructure option can address all of the flood risk in the Hawkesbury-Nepean Valley. As a result, the Strategy is a commitment to ongoing review and management of flood risk.

  1. What does the Hawkesbury-Nepean Flood Risk Management Strategy recommend?

The Strategy is a long term approach to managing existing and future flood risk in the Hawkesbury-Nepean Valley. The Strategy will operate under an adaptive management framework to take into account future unforseen changes such as greater urban growth and emerging technologies.

 The Strategy’s actions will be implemented in two phases:

  • Phase One will be delivered from 2016-2020 and focuses on actions that can be implemented in the short term. These include:
  • establishing the Hawkesbury-Nepean Flood Risk Management Directorate to coordinate the implementation of the Strategy actions across the key state organisations and local councils
  • developing and implementing a coordinated regional approach to flood risk land use, road and emergency planning
  • better information for the community about flood risk
  • improving evacuation road signage
  • developing an improved forecasting model to better support evacuation planning and operations
  • continuing ongoing improvements to flood emergency response and recovery arrangements
  • completing business cases for the priority local evacuation road infrastructure upgrades to enable access to major evacuation routes
  • completing the detailed design work, environmental impact assessment and consultation for planning approval and full business case for raising Warragamba Dam by around 14 metres.

The NSW Government will consider the final business case for raising the Warragamba Dam wall in 2019. Pending approval of the business case, and subject to environmental and planning approvals, it is expected to take three to four years to complete construction. The NSW Government will also consider the ongoing implementation, monitoring and improvement of the Strategy.

  1. What area does the Hawkesbury-Nepean Flood Risk Management Strategy cover?

The Strategy covers the Nepean and Hawkesbury river floodplains from Penrith to Sackville, covering approximately 365 km2. This area is subject to extensive, deep and prolonged flooding when large inflows into the Hawkesbury-Nepean Valley back up due to unique downstream topography, with confined sandstone gorges creating a ‘bath-tub’ effect, preventing flood waters escaping the Valley.

  1. Who will be responsible for implementing the Hawkesbury-Nepean Flood Risk Management Strategy?

The Hawkesbury-Nepean Flood Risk Management Directorate will be established initially within Infrastructure NSW to coordinate the implementation of the Strategy.

Strategy implementation will be a whole of government exercise involving many agencies and local councils in consultation with the community.

Local councils will remain responsible for flood risk management.

  1. How much will the Hawkesbury-Nepean Flood Risk Management Strategy cost to implement?

The NSW Government has allocated $58 million over the next four years (2016-17 to 2019-20) to implement Phase One of the Strategy.

A key component of Phase One will be planning and concept development works for raising the Warragamba Dam wall by around 14 metres.  $30 million has been allocated over four years to undertake detailed concept designs, environmental assessments and preparation of the full business case. 

$28 million has been allocated to establish a Hawkesbury-Nepean Flood Risk Directorate which will implement the Strategy and to deliver the non-infrastructure components of the Strategy. These include community engagement, evacuation signage, improved flood forecasting and integrating flood risk management with regional planning in the Hawkesbury Nepean Valley.

The implementation of the Strategy recommendations, including the raising of Warragamba Dam wall, is expected to cost around $800 million. The Warragamba Dam wall raising is estimated to cost about $700 million, subject to agreed final design, approval of the business case by the NSW Government in 2019 and environmental and planning approvals.

  1. Was climate change considered in developing the Hawkesbury-Nepean Flood Risk Management Strategy?

Yes. Understanding future climatic conditions is an important component of the flood modelling undertaken to assess likely future flood risk. The frequency of major flood events is influenced by the prevailing climatic conditions, so flood events were modelled under current and projected changed climate conditions to 2090. With projected climate change, the flood risk could increase. The Strategy will be periodically reviewed under an adaptive management framework to monitor and incorporate new information on climate change.

Flood Risk Basics

  1. Who is responsible for flood forecasting?

The Bureau of Meteorology is responsible for the national flood forecasting and warning service. It uses rainfall and streamflow observations, numerical weather predictions and hydrologic models to forecast and warn for possible flood events across Australia.

For more information see http://www.bom.gov.au/water/floods/

  1. What would happen if a flood occurred now in the Hawkesbury-Nepean Valley?

The NSW State Emergency Service (SES) is the lead response agency for flood emergency management across NSW. Under the SES Act of 1989, the SES has a responsibility to ensure that communities throughout the state are adequately prepared to reduce the impact of these events.

There is currently a NSW State Flood Plan for response to floods in NSW. This state-level plan is supported by a number of local level flood plans prepared in consultation with the individual local government councils.

In addition, in recognition of the major flood risk in the Valley the NSW SES has prepared the Hawkesbury-Nepean Flood Sub-Plan. The sub-plan establishes a framework for managing floods in the Valley, including allocating responsibilities to the agencies and organisations that manage the preparation for, response to and recovery from floods. These plans are regularly reviewed.

Information on flood emergency management and response can be found on the SES FloodSafe website – www.floodsafe.com.au

For emergency help in floods and storms call 132 500.

  1. How do I know if my house is in a flood prone area?

Under the NSW Government's Flood Prone Land Policy, management of flood prone land is primarily the responsibility of local councils. Homeowners can contact their local council to obtain flood information.

  1. What is a 1 in 100 year flood event and when will the next major flood event occur?

Understanding the likelihood of different sized floods occurring is important for managing flood risk.

The likelihood of a flood event can be described using a variety of terms. The term ‘1 in 100 year event’ refers to a flood level that has a 1 in 100 (or 1 percent) chance of occurring or being exceeded in any one year. For example, every year there is a 1 in 100 chance (or a ‘100 to 1’ chance) that there would be a flood in the Hawkesbury-Nepean Valley reaching a level of 17.3 metres or higher on the Windsor Bridge gauge. It means that a person living to 70 years of age has a 50% chance of experiencing this type of flood during their lifetime.

The occurrence of a flood of this, or any size, is completely random, and may occur several times or not at all in any 100-year period. History has shown that flood events such as these can happen numerous times within a single decade. While floods are defined by the chance they will occur in any one year, it is impossible to determine when the next major flood event in the Valley will occur.

  1. What are the contributing factors to floods?

The intensity and duration of rainfall are the primary cause of floods, and East Coast low pressures systems are the primary weather system behind large flood events in the Hawkesbury-Nepean Valley.

The magnitude of flooding created by rain falling on a catchment can also be influenced by other factors, including:

  • the land use in the catchment – vegetated areas slow down runoff and hence reduce flooding, while urbanized areas with sealed surfaces can increase flooding
  • pre-existing moisture level of the soil – dry catchments can soak up rainfall, while faster runoff from wetter catchments increases the risk of flooding
  • the presence of dams in the catchment – all dams reduce downstream flooding, even if not operated as flood mitigation dams, particularly if they were not full before the flood event
  • inflows from rivers and creeks outside the Warragamba Dam catchment – including the Nepean, Grose, Colo and South Creek.

Warragamba Dam flood mitigation

  1. Why does the Hawkesbury-Nepean Flood Risk Management Strategy recommend dam wall raising by 14 rather than 23 metres as recommended in 1995?

The 1995 option to raise Warragamba Dam wall by 23 metres was primarily to meet dam safety standards to ensure that the dam wall was safe to pass the worst possible flood, probable maximum flood (PMF), and secondarily for flood mitigation purposes. The final decision was to address dam safety and build a spillway to ensure that the dam wall is safe in all flood events. The challenge of flood mitigation was not addressed at this time.

The current recommendation to raise the dam wall by about 14 metres is for flood mitigation to address the flooding risk in the Valley. This dam wall raising will maintain dam safety and is assessed to be the most efficient and effective height for significant reduction to flood risk to life and property in the Valley. This height balances the need to reduce downstream flood risk and minimise the environmental impacts upstream.

  1. How does the 14 metre Warragamba Dam wall raising mitigate flood risk?

A 14 metre Warragamba Dam wall raising slows the floodwaters coming from the Warragamba catchment. This reduces the chance of regional flood events impacting the most highly populated area of the Valley, that is, areas impacted by floods with probabilities between 1 in 50 and 1 in 1000 chance per year.

The dam wall raising will significantly reduce most of the flood risk, including the floods on record, but not eliminate it completely.

It is not possible to build a dam high enough to capture such extreme, rare floods from the Warragamba catchment. Also, flooding from other catchments such as the Nepean, Grose, Colo and South Creek can contribute to downstream flooding, albeit generally smaller volumes compared to the Warragamba catchment.

Nonetheless, a raised dam wall at Warragamba will provide mitigation benefits by delaying and reducing the flood peak for all flood events, allowing more certainty of time for people to evacuate, protecting lives and reducing damages.

The raised dam will reduce the chance that floods reach houses and businesses. Maintaining current residential land use development controls, the proposed 14 metre dam wall raising is expected to reduce flood damages by about 75% on average.

  1. Would raising the Warragamba dam wall by more than 14 metres eliminate flood risk in the Valley?

Modelling shows that the optimal height to maximize flood mitigation benefit is about 14 metres.  No dam wall raising could be constructed to eliminate all flood risk in the Valley. While flows from Warragamba Dam are involved in all major regional flood events that pose the greatest risk to life and property, localised and low level regional flooding will still occur in the Valley. This is due to rainfall in areas outside the Warragamba Dam catchment. For example, in May 2016 and August 1986 the comparatively low flood peak at Windsor was caused by inflows from the Nepean River, Grose River and South Creek with no initial contribution from Warragamba Dam.

  1. What are the next steps in the dam raising, how long will it take to complete and what opportunities are there for consultation?

An environmental impact statement, including public consultation, to obtain environmental and planning approvals for a dam wall raising of around 14 metres for flood mitigation will be prepared by 2019. A final business case will also be prepared for consideration by the NSW Government in 2019, once the approvals are granted.

Construction can only begin when the environmental and planning approvals are granted. Construction is estimated to take 3 to 4 years.

Next steps include:

  • Detailed investigations and final concept design that will identify the construction cost estimate
  • detailed assessment of the potential environmental impacts from construction and operation of the proposed dam wall raising by 14 metres for flood mitigation. The environmental and planning approval for the dam wall raising for flood mitigation will be referred to the Commonwealth government under the Environment Protection and Biodiversity Conservation Act 1999
  • community and stakeholder consultation as part of the business case development and environmental impact assessment.

  1. What other major options to reduce flood risk were considered aside from raising the Warragamba Dam wall?

A number of options including compulsory house acquisition, building major new evacuation roads, construction of a new dam, or changing the course of the river through dredging, levees or diversion channels were assessed, but the risk to life could not be reduced as efficiently and effectively as raising the dam wall.

  1. How will a raised Warragamba flood mitigation dam affect the conservation, world heritage and special values (Native Title) upstream?

A raised dam wall for flood mitigation would result in temporarily holding back the floodwaters upstream until the water could be released in the following days, or for one or two weeks in rare floods. The impacts and options for reducing these impacts will be assessed and subject to public consultation as part of the environmental impact assessment and approval process.

  1. Why doesn’t Warragamba Dam have a flood mitigation function now? Can the existing Warragamba Dam be operated to mitigate floods?

Warragamba Dam is owned and operated by WaterNSW as a water supply dam and is not currently approved to operate as a flood mitigation dam. The dam provides around 80% of the greater Sydney metropolitan water supply.

The Taskforce investigated the feasibility of flood mitigation options by operating the current Warragamba Dam differently.  It found that these options were not as cost effective as dam wall raising and did not sufficiently address the risks posed by flooding. These options have limited flood mitigation effectiveness, with only small flood events mitigated and negligible flood mitigation benefit for the larger floods that pose a significant risk to lives or property.

The options included:

  • pre-releasing water ahead of a predicted flood inflow
  • permanently lowering the full supply level to provide airspace for flood mitigation, or
  • changing the operation of the gates to temporarily hold back flood water – this is called surcharging.

  1. Why can’t you lower Warragamba Dam’s full water supply level permanently for flood mitigation and operate the Sydney desalination plant to offset reduced water supply?

To achieve moderate reduction in flood risk to life and property, the full water supply would need to be permanently reduced by 12 metres. This is equivalent to reducing the dam water storage by nearly 40% or one and a half years of water supply to Sydney. However, the cost to maintain water supply security to Sydney under this option exceeded the costs of dam wall raising as new sources of water supply would need to be built in addition to operating the Sydney desalination plant.

  1. Will there be a more consistent approach to the consideration of flood risk in land use planning decisions across the Valley?

Yes. It is not possible for individual local councils to manage the cumulative impact of growth across the Valley.  A regional land use planning approach is critical to managing the cumulative impact of growth.

The regional road evacuation network is a common network across the key council areas. Growth in one area can affect the capacity of individuals in other areas to safely evacuate.  Land use planning will need to adopt a regional approach, which will be informed by new region-wide flood and evacuation modelling and will be guided by the work being undertaken by the Greater Sydney Commission.

A priority action will be the development and implementation of a Valley-wide flood risk map and development controls will need to account for both regional evacuation capacity and new flood modelling.

  1. What are the implications of the proposed regional approach to land use planning? 

The NSW Government and the Greater Sydney Commission will have a role to play in informing land use planning in the Valley based on region-wide flood modelling and evacuation capacity. A region-wide view of infrastructure investment is also needed. The proposed regional approach to land use planning will draw on new region-wide modelling to help provide more certainty for where there are opportunities to develop across the Valley while accounting for flood risk and risk to life.

  1. What is the flood planning level?

The flood planning level determines the area within which flood related development controls apply. In NSW the recommended flood planning level is set at the 1 per cent annual exceedance probability (or a 1 in 100 chance of it occurring in a given year), unless the local council seeks and receives approval from the NSW Government to apply a different flood planning level due to exceptional flood risk.  It does not typically identify the entire extent of flood prone land.

All local councils in the Hawkesbury-Nepean Valley have based their flood planning level on the 1 in 100 year flood. However, the Taskforce has found that this level does not adequately deal with flood risk in areas that have large flood depths such as Hawkesbury-Nepean Valley.  A more risk based and regional approach is required to deal with the exceptional flood risk in this Valley.

  1. Why is the Strategy needed if we currently have flood related development controls within the Valley?

The ‘flood planning level’ for residential development is based on 1 in 100 year flood level, below which flood related development controls apply. The controls relate to life and property safety, including floor levels and building standards to reduce flood risk. This level is suitable for many NSW floodplains, where the difference in flood depth between the 1 in 100 chance per year flood and the largest possible flood that can occur (known as the Probable Maximum Flood or PMF) is relatively small, typically less than two metres.

However the Hawkesbury-Nepean Valley has a unique and extreme flood risk. For example, in the Richmond-Windsor region, the PMF is up to nine metres above the 1 in 100 year flood level. As a result, the Valley’s current land use planning controls for flood risk do not apply to areas in the Valley subject to significant flood risk.

  1. Will development occur after Warragamba Dam wall is raised?

Yes. However, some new development restrictions may apply particularly around areas with existing higher flood risk.

The 14 metre Warragamba Dam wall raising is designed to reduce flood risk for the current and future population. As the dam wall raising will not eliminate flood risk entirely, land use will need to be carefully managed in the Hawkesbury-Nepean Valley. The area subject to current flood related development controls based on the 1 in 100 year flood level will to be subject to those controls following the dam wall raising to ensure that the benefits from the dam wall raising are maintained. It is important to ensure that population growth in the Valley is carefully managed, both in terms of absolute numbers of people and the distribution of the population within the Valley. Both these things can impact on the region’s capacity to evacuate in the event of a major flood.  Investigations are ongoing about whether new development restrictions should apply in the Valley.

  1. What impact will this have on property values and insurance?

The implementation of the Strategy is aimed to reduce flood risk for people and properties in the Valley, but it is difficult to predict if and how property values may be affected. A recent review found that there was limited evidence that flood awareness or flood events had any influence on property prices (Yeo, Roche, and McAneney, 2015 FMA Conference).

The proposed raising of Warragamba Dam wall for flood mitigation will lead to a significant reduction in flood risk in the Valley. It is expected that a reduction in flood risk would be reflected in reduced flood insurance premiums, but the flood insurance premiums are determined independently by the insurance companies in accordance with their pricing policies.

Staying safe in a flood

Information on how to prepare for a flood can be found on the NSW State Emergency Services FloodSafe website

For emergency help in floods and storms call 132 500.


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